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Forest, Range & Wildland Soils

Spatial Distribution of Ammonium and Calcium in Optimally Fertilized Pine Plantation Soils

^a Dep. of Agronomy, Purdue Univ., Lilly Hall of Life Sciences, 915 West State Street, West Lafayette, IN 47907-2054
^b Dep. of Forestry and Natural Resources, Pfendler Hall, 715 West State St., West Lafayette IN 47907-2061
^c USDA Forest Service Southern Research Station, Forestry Sciences Lab., P.O. Box 12254, 3041 East Cornwallis Rd., Research Triangle Park, NC 27709

^* Corresponding author (andyg{at}purdue.edu)

Commercial timber production is increasingly reliant on long-term fertilization to maximize stand productivity, yet we do not understand the extent to which this practice homogenizes soil properties. The effects of 16 yr of optimal fertilization and optimal fertilization with irrigation (fertigation) on forest floor depth, pH, total organic carbon (TOC) and total nitrogen (TN) content, and concentrations of potentially plant available NH₄⁺, Ca²⁺, Mg²⁺, and Al³⁺ in the fine root zone of monoculture loblolly pine (Pinus taeda L.) stands in North Carolina were examined. Generally, optimal fertilization significantly increased forest floor depth and the concentrations of potentially plant available NH₄⁺, Ca²⁺, and Mg²⁺. No significant effects on TOC or TN were observed. However, differences were observed between optimally fertilized and fertigated treatments. Specifically, fertilization alone tended to retain more Ca²⁺ and Mg²⁺ in the root zone and to increase pH more than fertigation. Semivariogram analysis indicated optimal fertilization generally lead to a significant increase in the effective spatial autocorrelation ranges of NH₄⁺ and Ca²⁺ at the end of the growing season, from approximately 25 m in controls to more than 100 m. However, optimally fertilized and fertigated plots differed in terms of their NH₄⁺ and Ca²⁺ spatial autocorrelation ranges, with the ranges in fertigated plots being 1.5- to 2-fold greater. A comparison of kriged maps of NH₄⁺ and Ca²⁺ availability in fall 2000 and spring 2001 suggested that the spatial distribution of Ca²⁺, which has not been added to these soils for 6 yr, was stable, whereas the heterogeneity of NH₄⁺ distribution increased post fertilization.

Abbreviations: SETRES, Southeastern Tree Research and Education Site • TN, total nitrogen • TOC, total organic carbon